1. Field of the Present Invention
The present invention relates generally to vibratory mechanical actuation devices, and, in particular, to tunable machining tools utilizing a plurality of actuator devices.
2. Background
Actuator mechanisms capable of creating high-frequency, vibratory movement at very small, precise amplitudes are used in a variety of fields. Because of their unique properties, piezoelectric actuators are among the most widely used mechanisms for this type of actuator. For example, vibration assisted diamond turning, referred to herein as “VADT,” is a known technique that enables the use of diamond tools to machine brittle materials without excessive wear. Researchers are not sure why the technique reduces wear. Many postulate that the vibration motion allows the tool to leave the workpiece momentarily, allowing either cooling, the penetration of cutting fluids into the area of material removal, or other mechanisms. The systems apparently only work when the maximum linear velocity of the tool exceeds the linear velocity of the part being machined. In diamond turning applications, performance parameters on the order of 40 kHz vibrations with amplitudes of 6-8 micrometers are desired.
Current VADT systems have been in use for a number of years. However, developing a tool that can vibrate at those frequencies and amplitudes is non-trivial. Most commonly, known systems rely on small, piezoelectrically-driven motions to excite natural frequencies in a tool bar, resulting in amplification of the output displacement and creating the desired motion. However, previous ultrasonic vibration cutting systems, such as those disclosed in U.S. Pat. No. 5,218,893 to Shikata et al. and U.S. Pat. No. 4,911,044 to Mishiro et al., utilize tools having a fixed resonant frequency, and such systems are not easily modified to change either the amplitude or the frequency of tool motion. In addition, the response is affected by damping caused when the tool contacts the workpiece and begins to cut, thus making the system dynamics variable and nonlinear.
One significant drawback of previous actuator-driven systems such as VADT is the absence of a means for tuning the actuators. If an actuator could be tuned, then the mechanics of the material removal process could be more thoroughly examined through the ability to deterministically vary the vibration parameters. In addition, however, a tunable actuator would enable the machine or process to adapt the nature of the vibration to changing cutting conditions. For example, in diamond turning, since linear velocity is the product of the radius from the center of a workpiece times angular velocity, which is constant, the linear velocity of the cutting tool along the surface of the workpiece varies as the tool approaches the center of the workpiece. This means that the linear speed of the tool changes throughout the cutting operation. In prior art systems, which are resonant frequency-based systems, the frequency of operation cannot be easily modified and thus will remain constant throughout this dynamic cutting process. A tunable tool would allow compensation for the changes in linear velocity.
Examples of other applications in which stacks of piezoelectric actuators, driven at a high frequency by a single impulse signal, include U.S. Pat. No. 5,431,010 to Stone, which discloses a high speed, amplitude variable thrust control method; U.S. Pat. No. 6,273,538 to Mitsuhashi, which discloses a method of using a pulsed piezoelectric stack to drive an ink jet head; U.S. Pat. No. 6,052,251 to Mohajerani et al., which discloses an actuator arm integrated piezoelectric micro actuator; and U.S. Pat. No. 5,438,554 to Seyed-Bolorforosh et al., which discloses a tunable acoustic resonator for clinical ultrasonic transducers. However, no known system utilizes two or more impulse signals in concert to produce a higher output frequency.
The use of torsional flexure in other applications has been established. For example, U.S. Pat. No. 6,201,629 to McClelland et al. discloses the use of a torsional micro-mechanical mirror system, while U.S. Pat. No. 4,044,283 to Allison discloses the use of an electromechanical resonator. The use of a rotary arm based turning machine for ophthalmic lenses is also known. However, although it is well known that a rotational flexure generally has a higher natural frequency, and potentially lower inertia, than a translational flexure there are no elastic torsional flexures known which rely on these characteristics and use multiple actuator devices to produce frequencies greater than that achievable using a single actuator device.